1. Field of the Invention
This invention relates to a fabrication method of a power transmission apparatus having a rolling bearing journaling a wheel to which power from a drive source is transmitted, and the power transmission apparatus fabricated by the method.
2. Description of the Related Art
Generally, a compressor for the car air-conditioner transmits power from an external power source, such as an engine through a belt, a pulley, etc. Between the belt and the input shaft of the compressor, a power transmission apparatus capable of shutting off the power is disposed in case excessive torque is generated.
A power transmission apparatus 10 of this type, as shown in FIG. 9, includes a cylindrical bearing support portion 13 projected from a compressor housing 11, an input shaft 15 journalled on the inside of bearing support portion 13, a synthetic resin pulley 19 journalled on the outer periphery of bearing support portion 13 through a ball bearing 17 and rotationally driven by a drive source, and a power transmission device 21 interposed between pulley 19 and input shaft 15 to couple pulley 19 and input shaft 15 to each other, wherein a cylindrical metal ring 23 is fitted on the inner periphery of pulley 19. An outer ring 25 of ball bearing 17 is press-fitted into the inner periphery of metal ring 23, and fixed by caulking at six points 27 equidistantly in the peripheral direction on a front-side end surface 23a of metal ring 23.
Punch-caulked points 27 are formed using a punch 31 as shown in FIGS. 10 and 11. Punch 31 is a wedge-shaped tool having a predetermined peripheral width. The forward end of punch 31 is driven into front-side end surface 23a of metal ring 23 and the inner peripheral portion of front-side end surface 23a is pushed out radially inward thereby forming a protrusion 33. Protrusion 33 engages a front-side outer peripheral portion 25a of outer ring 25, and thus prevents ball bearing 17 from coming out toward the front side.
However, in the conventional method of caulking the ball bearing, protrusion 33 fails to positively press front-side outer peripheral portion 25a of outer ring 25, and therefore once the press-fitting of outer ring 25 on metal ring 23 is loosened, outer ring 25 may become displaced rearward and pulley 19 interfere with housing 11. Even if the press-fitting state is not loosened, pulley 19 may become displaced under a large force, which may be exerted in the axial direction on pulley 19.
To cope with this situation, a method is considered in which punch 31 is pushed into a further depth in the axial direction in order that protrusion 33 is positively pressed against front-side outer peripheral portion 25a of outer ring 25.
However, with this configuration, the base of protrusion 33 presses outer ring 25 excessively, as shown in FIG. 12, which results in a radially inward force being exerted on front-side outer peripheral portion 25a of outer ring 25, which is then subjected to a bending stress F. As a result, compressive stress is exerted on a ball track surface 25b of outer ring 25, and the roundness of the track surface is adversely affected. In view of the fact that front-side end portion 23b of metal ring 23 is expanded radially outward, synthetic resin pulley 19 may become deformed and cracked.
Japanese Unexamined Patent Publication No. 2006-153258 discloses a power transmission apparatus in which a wheel with power transmitted thereto from a drive source is journalled by a ball bearing, and a power transmission device is interposed between the wheel and the input shaft of the compressor to shut off the power in case an excessive torque is generated.